A conventional OFDM transmission/reception apparatus is explained with reference to FIG. 1 below. FIG. 1 is a block diagram illustrating a configuration of a conventional OFDM transmission/reception apparatus.
In FIG. 1, parallel-serial converter (hereinafter referred to as “P/S converter”) 101 inserts important information into transmission data. This important information refers to a kind of information normal communication of which is likely to be difficult to be maintained if the other end of communication has a poor error rate characteristic during reception. That is, the important information above is a kind of information requiring a better-error rate characteristic than other information (transmission data, for example).
An example of the important information above is retransmission information or control information, etc. Retransmission information refers to information retransmitted to the other end of communication according to a retransmission command issued by the other end of communication. Control information refers to information used for the user at the other end of communication to reliably receive an appropriate signal. Examples of control information can be information indicating bursts in a communication frame to be received by the other end of communication and information indicating the current modulation system during adaptive modulation, etc.
Serial-parallel converter (hereinafter referred to as “S/P converter”) 102 converts the transmission signal, which is the output of P/S converter 101, to a plurality (here 4) of signals.
Mapping circuits 103 carry out primary modulation on the signals from S/P converter 102 and send the primary-modulated signals to Inverse Fast Fourier Transform (hereinafter referred to as “IFFT”) circuit 104. IFFT circuit 104 performs inverse Fourier transform processing on the primary-modulated signals. D/A converter 105 converts the transmission signal, which is the output of IFFT circuit 104, to an analog signal.
On the other hand, A/D converter 106 converts a reception signal to a digital signal and sends it to Fast Fourier Transform (hereinafter referred to as “FFT”) circuit 107. FFT circuit 107 performs Fourier transform processing on the output signal of A/D converter 106.
Delay detectors 108 perform delay detection processing on the subcarriers obtained by Fourier transform and determination circuits 109 determine delay detection processing. P/S converter 110 converts a plurality of signals from determination circuits 109 to a single signal and S/P converter 111 extracts important information from the output of P/S converter 110.
Then, the transmission/reception operations of the conventional apparatus with such a configuration are explained.
After important information is inserted by P/S converter 101, the transmission data is converted to a plurality of signals by S/P converter 102. The plurality of signals from S/P converter 102 are subjected to primary modulation by mapping circuits 103. The primary-modulated signals are subjected to inverse Fourier transform processing by IFFT circuit 104. The signals subjected to inverse Fourier transform processing by IFFT circuit 104 are converted to a digital signal by D/A converter 105 and transmitted.
The reception signal is converted to an analog signal by A/D converter 106 and then subjected to Fourier transform processing by FFT circuit 107. The signals carried by subcarriers after Fourier transform processing are subjected to delay detection processing by delay detectors 108. The signals subjected to delay detection processing are determined by determination circuits 109 and sent to P/S converter 110. A plurality of signals from determination circuits 109 are converted to a single signal by P/S converter 110 and sent to S/P converter 111. S/P converter 111 extracts retransmission information and reception data from the single signal.
In this way, by the transmitter inserting important information into the transmission signal, and the receiver extracting important information from the reception signal and carrying out reception processing based on the extracted important information, the receiver can receive the signal transmitted by the transmitter correctly. This allows a smooth communication between the transmitter and receiver.
If retransmission information is taken as an example of important information, by the transmitter inserting retransmission information into the transmission signal and the receiver extracting the retransmission information from the reception signal, the receiver can send an appropriate retransmission command to the transmitter. That is, the receiver can send back a signal with information carried on a control channel indicating which cell of which burst had an error.
However, the conventional apparatus has problems as shown below. That is, in the conventional apparatus, as the transmission efficiency is improved, the channel quality deteriorates, and the more the transmission efficiency is improved, the higher the probability that the receiver will not correctly receive the signal (for example, important information and transmission data) sent by the transmitter is. That is, the higher the transmission efficiency, the worse the error rate characteristic of important information in the receiver. As a result, it will be difficult for the receiver to maintain correct reception and it will be difficult to maintain a normal communication between the transmitter and receiver as a whole.
Here, suppose the modulation system is changed from QPSK to 8 PSK to improve the transmission efficiency.
In 8 PSK, one symbol is expressed with 3 bits. As shown in FIG. 2, in the 1st bit, “0” and “1” are switched round every 180 degrees on an I-Q plane; in the 2nd bit, “0” and “1” are switched round every 90 degrees on the I-Q plane; and in the 3rd bit, “0” and “1” are switched round every 45 degrees on the I-Q plane. That is, every time the number of bits increases, the phase likelihood becomes half the phase likelihood of the preceding bit. Therefore, the phase likelihood of the 3rd bit becomes half the QPSK phase likelihood, and thus errors occur most frequently in the 3rd bit.
Here, when retransmission information is used as important information if the error rate characteristic of retransmission information in the receiver deteriorates as described above, the transmitter retransmits this retransmission information more frequently, which makes longer the time until the communication is completed. Normally, there is a limit to the number of retransmission times of certain information and if retransmission is not completed within this limit, error correction is not carried out for this information. This makes it impossible to maintain a normal communication when carrying out a communication, which requires an optimal error characteristic such as image communication.
Moreover, when information indicating the current modulation system is used as important information, if the error rate characteristic of this information deteriorates on the receiving side, it is difficult for the receiving side to recognize the modulation system used by the transmitting side, making it impossible for the receiver to receive the signal transmitted by the transmitter. This makes it impossible to maintain a normal communication between the transmitting side and receiving side.